The present disclosure relates, in various embodiments, to multilayer electrodes and thin-film transistors (TFTs) comprising the same.
TFTs are fundamental components in modern-age electronics, including, for example, sensor, imaging, and display devices. TFT circuits using current mainstream silicon technology may be too costly for some applications, particularly for large-area electronic devices such as backplane switching circuits for displays (e.g., active matrix liquid crystal monitors or televisions) and low-end electronic devices such as radio frequency identification (RFID) tags where high switching speeds and/or high density are not essential. The high costs of silicon-based TFT circuits are primarily due to the capital-intensive silicon fabrications as well as the complex high-temperature, high-vacuum photolithographic fabrication processes under strictly controlled environments needed to make them. Because of the cost and complexity of fabricating silicon-based TFT circuits using conventional photolithography processes, there has been an increased interest in organic TFTs (OTFTs). Organic materials offer not only the possibility of using low-cost solution or liquid fabrication techniques, but also attractive mechanical properties such as being physically compact, lightweight, and flexible.
OTFTs are generally composed of, on a substrate, an electrically conductive gate electrode, source and drain electrodes, an electrically insulating gate dielectric layer which separate the gate electrode from the source and drain electrodes, and a semiconducting layer which is in contact with the gate dielectric layer and bridges the source and drain electrodes. Their performance can be determined by the field effect mobility and the current on/off ratio. High mobility and high on/off ratio are desired.
Both the mobility and on/off ratio are affected by the total resistance between the source and drain electrodes, Rtotal. If the total resistance is high, then high electrical field strengths are necessary to inject and extract charge carriers. The total resistance can be determined using the formula:Rtotal=Rcontact+Rsc Rcontact is the contact resistance at the interface of each electrode and the semiconductor layer. Rsc is the resistance in the length of the semiconductor layer between the source and drain electrodes.
One way to lower the total resistance is to lower Rsc by reducing the semiconductor channel length between the source and drain electrodes. This increases mobility, but reduces the on/off ratio if the Rcontact is very low. This limits the application of such a TFT.
Another way to lower the total resistance is to reduce the contact resistance. Contact resistance is generally minimized by selecting an electrode material having a work function identical or very close to energy level of the semiconductor. The energy level is the highest occupied molecular orbital (HOMO) of the semiconductor in the case of p-type semiconductor or the lowest unoccupied molecular orbital (LUMO) of the semiconductor in the case of n-type semiconductor. If contact resistance is minimized when Rsc is low, however, the current on/off ratio decreases because the TFT exhibits high off current.